The present invention relates to methods and apparatus for providing image data from which an image of at least a portion of a target object may be generated. In particular, embodiments of the invention relate to methods and apparatus which reduce a time taken to record data for producing the image data.
WO 2005/106531, which is incorporated herein by reference for all purposes, discloses a method and apparatus of providing image data for constructing an image of a region of a target object. Incident radiation is provided from a radiation source at the target object. An intensity of radiation scattered by the target object is detected using at least one detector. The image data is provided responsive to the detected radiation. A method for providing such image data via an iterative process using a moveable softly varying or bandwidth limited probe function such as a transmittance function or illumination function is also disclosed. The methods and techniques disclosed in WO 2005/106531 are referred to as a ptychographical iterative engine (PIE).
PIE provides for the recovery of image data relating to at least an area of a target object from a set of diffraction pattern measurements. Several diffraction patterns are recorded at a measurement plane using one or more detectors, such as a CCD or the like. A probe function, which might be a transmittance function associated with a post-target object aperture or an illumination function, must be known or estimated.
WO 2010/064051, which is incorporated herein by reference for all purposes, discloses an enhanced PIE (ePIE) method wherein it is not necessary to know or estimate the probe function. Instead a process is disclosed in which the probe function is iteratively calculated step by step with a running estimate of the probe function being utilised to determine running estimates of an object function associated with a target object.
Other methods of providing image data based on measurement of scattered radiation are also known.
FIG. 1 illustrates an apparatus 100 suitable for use in the PIE and ePIE methods referred to above, and other coherent diffractive imagine techniques. The apparatus 100 is suitable to provide image data of an object which may, although not exclusively, be used to produce an image of at least a region of the object.
A radiation source, which although not shown in FIG. 1, is a source of radiation 10 which falls upon a focusing arrangement 20, such as one or more lenses, and is caused to illuminate a region of a target object 30. It is to be understood that the term radiation is to be broadly construed. The term radiation includes various wave fronts. Radiation includes energy from a radiation source. This will include electromagnetic radiation including X-rays, emitted particles such as electrons. Other types of radiation include acoustic radiation, such as sound waves. Such radiation may be represented by a wave function Ψ(r). This wave function includes a real part and an imaginary part as will be understood by those skilled in the art. This may be represented by the wave function's modulus and phase. Ψ(r)* is the complex conjugate of Ψ(r) and Ψ(r)Ψ(r)*=|Ψ(r)|2 where |Ψ(r)|2 is an intensity which may be measured for the wave function.
The lens 20 forms a probe function P(r) which is arranged to select a region of the target object 30 for investigation. The probe function selects part of an object exit wave for analysis. P(r) is the complex stationary value of this wave field calculated at the plane of the object 30.
It will be understood that rather than weakly (or indeed strongly) focusing illumination on the target object 30, unfocused radiation can be used with a post target aperture. An aperture is located post target object to thereby select a region of the target 30 for investigation. The aperture is formed in a mask so that the aperture defines a “support”. A support is an area of a function where that function is not zero. In other words, outside the support, the function is zero. Outside the support the mask blocks the transmittance of radiation. The term aperture describes a localised transmission function of radiation. This may be represented by a complex variable in two dimensions having a modulus value between 0 and 1. An example is a mask having a physical aperture region of varying transmittance.
Incident radiation 10 thus falls upon the up-stream side of the target object 30 and is scattered by the target object 30 as it is transmitted. The target object 30 should be at least partially transparent to incident radiation. The target object 30 may or may not have some repetitive structure. Alternatively the target object 30 may be wholly or partially reflective in which case a scattering pattern is measured based on reflected radiation.
A specimen wave O(r) is thus formed as an exit wave function of radiation after interaction with the object 30. In this way O(r) represents a two-dimensional complex function so that each point in O(r), where r is a two-dimensional coordinate, has associated with it a complex number. O(r) will physically represent an exit wave that would emanate from the object which is illuminated by a plane wave. For example, in the case of electron scattering, O(r) would represent the phase and amplitude alteration introduced into an incident wave as a result of passing through the object 30 of interest. The probe function P(r) (or transmission function) selects a part of the object exit wave function for analysis. It will be understood that rather than selecting an aperture a transmission grating or other such filtering function may be located downstream of the object function. The probe function P(r-R) is an aperture transmission function where an aperture is at a position R. The probe function can be represented as a complex function with its complex value given by a modulus and phase which represent the modulus and phase alterations introduced by the probe into a perfect plane wave incident up it.
An exit wave function ψ(r,R) is an exit wave function of radiation 35 as it exits the object 30. This exit wave ψ(r,R) forms a diffraction pattern Ψ(u) at a diffraction plane. Here r is a vector coordinate in real space and u is a vector coordinate in diffraction space.
In order to select the region of the target object 30 to be illuminated or probed, the lens(es) 20 or aperture may be mounted upon an x/y translation stage which enables movement of the probe function with respect to the object 30. It will also be realised that the object 30 may be moved with respect to the lens(es) or aperture.
A detector 40 is a suitable recording device such as a CCD camera or the like which allows the diffraction pattern to be recorded. The detector 40 allows the detection of the diffraction pattern in the diffraction plane. The detector 40 may comprise an array of detector elements, such as in a CCD.
As will be appreciated, in order to produce image data corresponding to the target object, such as the object function O(r), a plurality of diffraction patterns are recorded at corresponding, partly overlapping, probe positions.
It is an object of embodiments of the invention to at least mitigate one or more of the problems of the prior art.